The 61-year-old was sacked from the Stadium of Light post on Saturday evening, hours after the Wearsiders' 1-0 home defeat to Manchester United.
Sunderland had gone eight games without a win in the Barclays Premier League under O'Neill before chairman Ellis Short opted to dispense with his services and take the controversial decision to replace him with Paolo Di Canio.
It was a bold move given the former Aston Villa and Celtic boss had inspired a team bereft of confidence and languishing in 16th place when he took over in December 2011, guiding them to a 13th-placed finish.
O'Neill would not be drawn on the appointment of the Italian but he was clear about his feelings after his tenure was cut short.
O'Neill told Radio 5 Live: "I am still pretty disappointed, frustrated as much as anything else but life goes on.
"I'm in the business now where I think very little shocks you about professional football especially in the last 10 years. You can lose a job in management if your tie doesn't fit your suit.
"With the experience I have had over the years...also coming into the football club at a time when the club was pretty well on its knees. I believe I saved it from relegation last year and I thought that opportunity should still have been afforded to me."
Di Canio's reign started amid much controversy with Labour MP David Miliband quitting as club vice-chairman in protest at political views the Italian has previously espoused. It also prompted opposition from a number of fans' groups, while the Dean of Durham wrote an open letter.
While the controversy may have been slightly ameliorated by a personal statement by Di Canio, it was not the start the club would have wanted as they prepare for a relegation battle, starting at Chelsea on Sunday.
O'Neill, though, would not be drawn on Di Canio's arrival at the Stadium of Light, saying: "I've absolutely nothing to say at all about that.
"That is part of Sunderland's future now and that is Sunderland's prerogative to do what they want."
Of Di Canio's prospects of keeping the Black Cats up despite having no top-flight managerial experience, he added: "He expects to keep Sunderland up in the division and that is fine. He thinks he'll be able to do it and that is why he has been brought in.
"He has said he will do it and that is up to him."
The former Leicester and Wycombe boss also reiterated his belief that Sunderland did not have enough quality through the team.
"The DNA of Sunderland Football Club is about passion - it has been throughout the years and that is very important, very important to the supporters of the football club.
"It carries you a certain distance (but) of course you have to have ability to play.
"Overall we've got some very fine players at Sunderland at this moment.
"But we haven't enough true ability throughout the side to be able to cope with every single thing that is thrown at us.
"I don't think it is a major criticism, it is there for all to see, it's actually a fact.
"It's something you have to try and cure and want to rectify as soon as possible - that is not always possible given the time."
O'Neill also praised the older managers in the game, underlining the virtues of experience.
"You talk about a dinosaur.
"The biggest dinosaur in the game happens to be the best manager in the game and he has been for years.
"He'll go down in history as one of the greats and I am talking about Sir Alex Ferguson and there is no bigger dinosaur, he is 70 years of age, he has gone with the times.
"It's interesting England turned to Roy Hodgson to become their manager with all that vast experience of international football and club football all over Europe and they turned to him because he has got this experience.
"But not only that - he has been involved in the game for a long time and he has got the know-how.
"QPR turned to Harry Redknapp. Everybody has a shelf life but it doesn't depend on age."